Help with due due diligence before buying property

Hello Everyone,

I am planning to come to Istanbul, identify an apartment, buy, and then relocate my family. I have contacted a well-established company (it is one of the biggest that appears on your google search for buying properties in turkey). They also happen to have their own lawyer who normally helps with the process and due diligence.

i am just looking for some quite knowledgeable about Istanbul property buying process, and prices (in comparison to the market value) so that I am not taken advantage of. to be specific, I would need help with the following:

1) check the company and their lawyers are registered and genuine (from their online profile and testimonials, they seem reputable). Check their lawyers (subsidiary law firm) is registered with Bar association as I might need to sign a power of attorney with them.
3) Double-check builders (construction companies) they are dealing with (especially for flats that I am interested to buy)
2) cross-check prices listed against the market value (ensure it isn't a rip-off ).

I am not looking for another real estate agency or lawyer as they already have those, i just need someone that can help with background checks, etc.